Vol 118, No 6 (2017)

In the articles of Bkhadeshia, a new class of high-strength steels based on the structure of carbidefree bainite with an enhanced carbon content has been developed. According to Bkhadeshia, the main factor responsible for the high solubility of carbon is the occurrence of a tetragonality of the bainite lattice. To check this effect, in this article, the theory of tetragonality of martensite of iron alloys developed by Zener and Khachaturyan was applied to bainite under the assumption that the precipitation of carbides is prohibited. Equations for the chemical potentials of carbon and iron in austenite and in tetragonal ferrite have been derived. The equilibrium of these phases has been considered, and the calculations of the boundary concentrations of carbon and iron at different temperatures (300–1000 K) and at different parameters of the deformation interaction λ₀ have been performed. The rigorous calculations confirmed Bkhadeshia’s hypothesis that the suppression of the carbide formation during the formation of bainite leads to an increase in the carbon solubility in the bcc phase.

It is necessary to theoretically evaluate the thermodynamic properties of metallic nanoparticles due to the lack of experimental data. Considering the surface effects and crystal structures, a simple theoretical model is developed to study the size dependence of thermodynamic properties of spherical metallic nanoparticles. Based on the model, we have considered Co and Cu nanoparticles for the study of size dependence of cohesive energy, Au and Cu nanoparticles for size dependence of melting temperature, and Cu, Co and Au nanoparticles for size dependence of Debye temperature, respectively. The results show that the size effects on melting temperature, cohesive energy and Debye temperature of the spherical metallic nanoparticles are predominant in the sizes ranging from about 3 nm to 20 nm. The present theoretical predictions are in agreement with available corresponding experimental and computer simulation results for the spherical metallic nanoparticles. The model could be used to determine the thermodynamic properties of other metallic nanoparticles to some extent.

Frequency changes in the experimental dependences of the impedance on the strength of the magnetic-bias direct current flowing through a wire of magnetically soft amorphous alloy of composition Co₆₆Fe₄Nb_2.5Si_12.5B₁₅ with a low positive constant of saturation magnetostriction are due to the combined type of its magnetic anisotropy and indicate the presence of a multidomain structure with a helicoidal orientation of magnetization in the subsurface layer and also the effect of the motion of domain walls on the highfrequency impedance.

The temperature dependences of the electrical resistivity and thermal electromotive force (thermal e.m.f.) of the Ni–25 at % V, Ni–28 at % V, and Ni–33 at % V alloys in a temperature range of 300–1600 K have been reported; the dependences have been measured during slow heating and cooling of quenched and annealed samples. It has been shown that, near the order–disorder phase-transformation temperature, the temperature dependences of the electrical resistivity of the Ni₇₅V₂₅ and Ni₆₇V₃₃ alloys demonstrate a kink (second-order phase transition) and a jump (first-order phase transition), respectively. The behavior of the experimental dependences is discussed in terms of the band Mott s–d scattering model.

The effects of the substitution of nickel for iron on the magnetic properties, uniaxial anisotropy induced by magnetic field, and crystallization process of the nanocrystalline Fe_72.5–xNi_xCu_1.1Nb_1.9Mo_1.5Si_14.3B_8.7 FINEMET alloys modified with molybdenum have been studied. Nickel additions of up to 3 at % have been shown to increase the Curie temperature and magnetization of the alloy, whereas nickel additions of more than 6 at % lead to the decrease in these parameters as compared to those for the initial nickel-free alloy. Nickel additions of 6‒7 at % cause a linear increase in the induced uniaxial magnetic anisotropy constant. Nickel additions of more than 6 at % restrain the crystallization of the alloy and lead to substantial decrease in its magnetic hysteresis characteristics.

Al–Fe alloys prepared by casting, rapid quenching from the melt, and mechanical alloying from elemental powders have been studied using X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements in the initial state and after severe plastic deformation by high-pressure torsion using Bridgman anvils. The relationship between the phase composition, microstructure, and the microhardness of the investigated alloys has been established.

High-entropy alloys CoCrFeNiMnV_{Kharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108} (Kharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108 = 0.25, 0.5, 0.75, 1) were prepared by vacuum arc melting. The structure and microhardness of the alloys have been studied in the cast state and after annealing at temperatures of 700–1100°C. It has been found that the alloys consist of the fcc (γ) solid solution and intermetallic sigma (σ) phase. The volume fraction of the σ phase increases with increasing vanadium content. As a result of annealing, phase transformations occur, including the precipitation of σ particles from the γ phase and, vice versa, the precipitation of γ particles from the σ phase. It has been shown that the change in the volume fraction of the σ phase upon annealing occurs due to the changes in the total content of σ-forming elements, chromium and vanadium, in accordance with the lever rule. With increasing temperature, the volume fraction of the σ phase varies nonmonotonically; first, it increases, then it decreases. The microhardness of the alloys correlates well with the change in the volume fraction of the σ phase. The mechanisms of the phase transformations and quantitative relationships between chemical and phase compositions of the alloys and their hardness are discussed.

The effect of stresses on the microstructure and dispersed particles in a heating-performance Fe‒0.12C–0.06Si–0.04Ni–0.2Mn–9.5Cr–3.2Co–0.45Mo–3.1W–0.2V–0.06Nb–0.005B–0.05N (wt %) steel has been studied under long-term strength tests at Т = 650°C under initial applied stresses ranging from 220 to 100 MPa with a step of 20 MPa. Under an applied stress of 160 MPa, which corresponds to a time to fracture of 1703 h, a transfer from short- to long-term creep takes place. It has been shown that alloying with 3% Co and an increase in W content to 3% significantly increase the short-term creep resistance and slightly increase the long-term strength upon tests by more than 10⁴ h. The transfer from short- to the long-term creep is accompanied by substantial changes in the microstructure of the steel. Under long-term creep, the solid solution became depleted of tungsten and of molybdenum down to the thermodynamically equilibrium content of these elements in the solid solution, which leads to the precipitation of a large amount of fine particles of the Laves phase at the boundaries of laths and prior austenitic grains. At a time to fracture of more than 4 × 10³ h, the coalescence of the M₂₃С₆ carbides and Laves-phase particles occurs, which causes the transformation of the structure of fine tempered martensite lath structure into a subgrained structure.

Changes in the deformation behavior of steel solids and their properties have been considered after different methods of surface treatment (carburizing, nitriding, bombardment with low-energy ions, epilam application). Distinctions between concepts “structural strength of a material” and “structural strength of a workpiece” have been illustrated. It has been shown that, at the same material structural strength, the metal mechanical characteristics of a finished metal workpiece (i.e., the workpiece’s structural strength) change cardinally depending on the genesis of the modified layer (features of the structure that arise at the surface) and its contribution to the general state of the workpiece. After ion bombardment (layer thickness less than 1 μm) for the same material with the full retention of its structural strength, we can obtain in workpieces of the material either a very high (25–40%) strengthening without reducing plasticity or huge growth in the plasticity (increase in the elongation by a factor of 1.6) with enhanced strength. The effect is due to the nondislocation mechanism of plastic deformation of the surface layer nanostructurized upon ion bombardment and competition between strengthening and plasticizing depending on the magnitude of its contribution. The effectiveness of the strengthening action of ion bombardment is shown on connecting rod bolts 10 mm in diameter; the plasticizing effect is observed on thin sheet cold-rolled steels (improved stampability).